The present invention relates to a process of operating a multi-stage, isothermal reactor for the preferential oxidation of carbon monoxide in a hydrogen-rich fuel stream for a fuel cell.
One type of fuel cell uses hydrogen as a fuel and oxygen (as air) as an oxidant. The hydrogen used in the fuel cell can be derived from the reformation of methanol or other organics (e.g. hydrocarbons). For example, in the methanol reformation process, methanol and water (as steam) are ideally reacted in a catalytic reactor (a.k.a. xe2x80x9creformerxe2x80x9d) to generate a reformate gas comprising hydrogen and carbon dioxide according to the reaction:
CH3OH+H2Oxe2x86x92CO2+3H2
One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh et al, issued May 17, 1987, the disclosure of which is hereby incorporated by reference. Unfortunately, the reformats exiting the reformer also contains undesirably high concentrations of carbon monoxide most of which must be removed to prevent poisoning of the catalyst of the fuel cell""s anode. In this regard, carbon monoxide (i.e., about 1-3 mole %) contained in the H2-rich reformate/effluent exiting the reformer must be reduced to very low nontoxic concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode.
It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a methanol reformer can be reduced by utilizing a so-call xe2x80x9cshiftxe2x80x9d reaction wherein water (i.e. steam) is added to the methanol reformate/effluent exiting the reformer, in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate according to the following ideal shift reaction:
CO+H2Oxe2x86x92CO2+H2
Some (i.e., about 0.5 mole % or more) CO still survives the shift reaction, and any residual methanol in the reformate is converted to carbon dioxide and hydrogen in the shift reactor. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water and carbon monoxide.
The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it to the fuel cell. It is known to further reduce the CO content of H2-rich reformate exiting the shift reactor by a so-called xe2x80x9cPrOxxe2x80x9d (i.e., preferential oxidation) reaction effected in a suitable PrOx reactor operated at temperatures which promote the preferential oxidation of the CO by air in the presence of the H2, but without consuming/oxidizing substantial quantities of the H2 or triggering the so-called xe2x80x9creverse water gas shiftxe2x80x9d (RWGS) reaction. The PrOx process is described in a paper entitled xe2x80x9cMethanol Fuel Processing for Low Temperature Fuel Cellsxe2x80x9d published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in Vanderborgh et al U.S. Pat. No. 5,271,916, inter alia, the disclosures which are hereby incorporated by reference.
Desirably, the O2 required for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O2 exceeds about two times the stoichiometric amount needed, excessive consumption of H2 results. On the other hand, if the amount of O2 is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation will occur. Accordingly in practice, many practitioners use about 4 or more times the stoichiometric amount of O2 than is theoretically required to react with the CO.
PrOx reactors may be either (1) adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO). or (2) isothermal (i.e. where the temperature the reactor is maintained substantially constant during oxidation of the CO). The adiabatic PrOx process is sometimes effected via a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, because if the temperature rises too much, the RWGS reaction can occur which counter productively produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the RWGS reaction if (1) the reactor temperature can be kept low enough, and (2) O2 depletion prior to the end of the reactor can be avoided.
In the PrOx reactor there are three main chemical reactions: 1) CO oxidation, CO+0.5 O2xe2x86x92CO2; 2) H2 oxidation, H2+0.5 O2xe2x86x92H2O; and 3) Reverse-Water-Gas-Shift (RWGS) reaction, CO2+H2←xe2x86x92H2O+CO (RWGS). Reaction 1 is desired because it removes CO. Reaction 2 results in a loss of H2, thus reducing efficiency and so it is undesired. Reaction 3 not only consumes H2, but the reaction also results in the formation of CO, so it should be avoided. Reaction 1 and 2 directly compete for O2 and are exothermic, with reaction 1 being more exothermic. Reaction 3 is an equilibrium reaction and mainly occurs after all the oxygen has been consumed, or as indicated above when the relative amount of O2 with respect to the stoichiometric requirements has been substantially reduced. Reaction 3 is dependent on CO concentration and temperature (low CO and high temperature are more favorable for reaction 3) and thus good temperature control is essential.
Isothermal PrOx reactors and systems have been shown to meet the CO requirements (to prevent fuel cell stack poisoning) for both methanol and xe2x80x9cgasolinexe2x80x9d systems during normal operations (near steady state). However, during turndown situations (i.e., when the electrical load demand on the fuel cell stack has been substantially reduced), prior art PrOx reactor system have had some difficulty maintaining the CO concentration to acceptable levels due to increased residence time of the H2-rich gas stream in the PrOx reactor, leading to increased RWGS reaction and higher CO levels. This can be overcome by utilizing more excess O2, but as mentioned above the system efficiency is lowered.
The present invention overcomes some of the prior art deficiencies of operating a preferential oxidation reactor, particularly during turndown situations.
The present invention includes a process for controlling the carbon monoxide concentration out of a preferential oxidation reactor during turndown using staged multi-port air injection in a multi-stage, isothermal, carbon monoxide preferential oxidation (PrOx) reactor. The reactor preferably including a plurality air injectors, positioned along the length of the gas flow path through the reactor, for injecting varying amounts of air at each injector depending on the electrical load demand on an associated fuel cell stack, or depending on the concentration of CO exiting the reactor. During normal or peak operating conditions of the fuel cell stack, a majority of the air is injected through injectors positioned closer to the reformate gas inlet to the reactor. Upon a turndown of the electrical load demand on the fuel cell stack, a majority of the air is injected through the injectors more distant from the inlet and closer the to reactor outlet thus eliminating or substantially reducing the reverse water gas shift reaction, and substantially reducing the reaction of oxygen and hydrogen flowing through the PrOx.
The present invention includes a process of operating a multi-stage, isothermal, PrOx reactor for the selective reaction of CO with O2 in a H2-rich gas that flows through the reactor in order to reduce the CO content of the gas to a suitable level which is not toxic (i.e. below about 20 ppm) to a fuel cell catalyst. The reactor comprises a plurality of catalyzed heat exchangers serially arranged within a housing in the direction that the H2-rich gas flows through the reactor. The several catalyzed heat exchangers promote the CO+O2 reaction in a series of progressive steps in which the CO content of the gas is incrementally reduced from one catalyzed heat exchanger to the next as the gas flows through the catalyzed heat exchangers. The heat exchangers each comprise a plurality of thermally conductive barriers that separate the heat exchanger into (1) a plurality of first channels through which separate streams of the H2-rich gas flow, and (2) a plurality of second channels through which a gaseous or liquid coolant flows to maintain the temperature of the heat exchanger substantially constant. Preferably, the channels are constructed and arranged such that the direction the coolant flows in the second channels is transverse the direction of H2-rich gas flow in the first channels. Preferably, a single barrier separates a plurality of first channels from a single second channel, and most preferably, these first channels include a first channel from at least two different heat exchangers. The first channels have inlet and outlet ends for respectively admitting and exhausting the H2-rich gas into and out of the first channels. The barriers separating the first and second channels each have (1) a catalyzed first surface confronting a first channel for promoting the CO+O2 reaction therein, and (2) a second surface confronting a second channel for contacting the coolant in the second channel to extract heat from the catalyzed first surface, through the barrier, and maintain a substantially constant heat exchanger temperature that encourages the CO+O2 reaction and discourages the formation of CO from the reaction of CO2 with H2 (i.e. the xe2x80x9creverse water gas shift reactionxe2x80x9d).
The reactor includes a mixing chamber between each of the heat exchangers. The mixing chamber communicates with the outlet ends of the first channels of the heat exchanger that is immediately upstream of the chamber and the inlet ends of the heat exchanger that is immediately down stream of the chamber. The mixing chambers will preferably be defined by the housing enclosing the heat exchangers, and serve to receive and substantially homogenize the H2-rich gas streams exiting the upstream heat exchanger so as to distribute any unreacted O2 and CO in those streams substantially uniformly throughout the gas before it enters the downstream catalyzed heat exchanger. So distributing the O2 and CO intermediate the heat exchangers insures that more of the CO and O2 will contact the catalyzed surface of the downstream heat exchanger(s) and be consumed in the reactor. In a preferred embodiment of the invention, each mixing chamber includes at least one air inlet (or injector) for receiving at least a portion of the O2 required for the CO+O2 injection into the chamber for mixing with the streams exiting from the upstream heat exchanger before they enter the downstream heat exchanger. During normal or peak operating conditions of the fuel cell stack, O2 injection into the gas stream at various locations en route through the reactor promotes better consumption of the CO with less total O2, and insures that there will always be some O2 present in the H2-rich gas stream at the outlet end of the last catalyzed heat exchanger in the series to suppress the reverse water gas shift reaction that could otherwise occur there in the absence of O2.
According to an embodiment of the invention, the first surface of one of the barriers and the first surface of the next adjacent barrier are spaced from each other by at least one divider and together with the divider define first channels for at least two heat adjacent exchangers. These first channels of the two adjacent heat exchangers are most preferably substantially coplanar with each other, and are arranged and constructed to cause the gas therein to flow in opposite directions to each other.
The heat exchangers may be aligned end-to-end along an axis of the housing such that the outlet end of the upstream heat exchanger directly opposes the inlet end of the heat exchanger downstream of the chamber. Alternatively, the heat exchangers may be aligned side-by-side along an axis of the housing such that the inlet end of one heat exchanger is substantially coplanar with the outlet end of the next adjacent heat exchanger, and the chamber adjoining the inlet and outlet ends is defined by the housing and is adapted to reverse the direction of flow of the gas exiting the outlet end and entering the inlet end. Inlet and exhaust manifolds at the first and last heat exchangers in the series respectively serve to supply H2-rich gas to the first heat exchanger and collect the gas from the last heat exchanger.
In a preferred embodiment, the present invention utilizes multiport air injection during normal or peak operating conditions of the fuel cell stack to improve the efficiency of the preferential oxidation of CO to CO2 and reduce or eliminate the reverse water gas shift reaction. During turndown situations, air injection at the front of the reactor is substantially reduced and air is injected through injectors more distant from the inlet of the reactor and closer to the outlet of the reactor to reduce or eliminate the reverse water can shift reaction during turndown situations wherein a hydrogen-rich gas is still flowing through the reactor.